Television system



June 2, 1942. A. v. BEDFORD TELEVISION SYSTEM 7 sheets-sheet 1 FiledDec, 18, 1937 :Snvmtor Bedford (Ittorneg June 2, 1942. A. v. BEDFORDTELEVISION SYSTEM Filed Dec. 18, 1957 7 Sheets-Sheet 2 nventor im@'uedfo nz Gttorneg June 2, 1942- A. v, BEDFORD n 2,284,714

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Delay and 'hafptng Circalls farlanhing 117 122 cagna/v7' +-l --l L '7sheets-sheet e A. V. BEDFORD TELEVISION SYSTEM Filed DeG. 18, `193'7 112-M1171 M- 10o l* 1a 294ai lua l l v 60N CIRCUITS F'OH .BLFNKNG' P l f l14o 16o :Sunentor ford l l i l 147 F2619. l V l M 'Hwa-V531 www@ June 2,1942. A. v. BEDFORD TELEVISION SYSTEM Filed Dec. 18, 1937 7 Sheets-Sheet7 Patented June 2, 1942 'rELEvIsIoN SYSTEM Alda.v V. Bedford,"Collingswood, N. J., assigner to `Radio Corporation oi America, acorporation oi Delaware Application December 18, 1937, Serial No.180,493

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g 8 Claims.

My invention relates to television systems and particularly to`thefeature of providing synchro- `msm between theV scanning at thetelevision transmitter and the scanning at the television receiver.

In television systems it is the usual practice to `maintain synchronismbetweenA the transmitter and the receiver by transmitting horizontalsynchronizing impulses at the end of each scanning l line and verticalsynchronizing impulses at the end of `each pictureframe, all thesynchronizing impulses being `combined with the picture signals and`transmitted as a composite signal.

In orderfthat the synchronizingsignals may be separated from the picturesignals at the re.4

ceiver, the synchronizing impulses aremade of f greater amplitude thanpicture signals of like polarity. Thus amplitude separation may beemployed by means ofl a separating or clipping" tube.

After the picture signals have been removed it is the usual practice toseparate the horizontal synchronizing impulses from the verticalsynchronizing `impulses either by amplitude selection or by frequencyselection.

When amplitude selection is employed it has been customary to transmitthe vertical synchronizing impulses with a greater `amplitude than thehorizontal synchronizing impulses or vicel versa.` While this methodgives very exact synchronizing, it is undesirable because itisuneconomical of the transmitter power, too much power being utilized for,synchronizing signals and not enough for picture signals.

Frequency `separation of horizontal synchronizing impulses and verticalsynchronizing impulses is satisfactory from the transmitter powerstandpoint but it has been found that it does not give as exactsynchronization as is desired, especially when the picture is beinginterlaced. This is due largely to the fact that, in order to obtainfrequency` separation, 4the vertical synchronizing impulses are of much'longer duration than the horizontal synchronizing impulses and that thevertical synchronizing impulses are supplied to an integrating circuitat the receiver whereby the desired separation results. One way ofimproving the operation of.a system of this type is described andclaimed in my United States Patent Noil 2,192,121, issued February 27,194,0, entitled Television systems and the method of operation thereof,"and assigned to the Radio Corporation of America.

An object of my invention is to provide Van improved method of and meansfor synchronizing scanning at the receiver.

the scanning at a picture `transmitter with the A further object of myinvention is to provide a method'of and means for synchronizing thetransmitter and receiver of `a television system which combine theadvantages of the above-mentioned methods of synchronizing impulsevselection or separation without their disadvantages.

A further object of my invention is to provide a method of and means forobtaining very exact synchronizing in a television system without beingwasteful of transmitter power.

A further object of my invention is to .provide4 a method of and meansfor obtaining complete isolation of horizontal synchronizing impulsesand vertical synchronizing impulses.

A still further object of my invention is to provide improved'interlacedscanning for television systems.

A still further object of my invention is to provide an improvedtelevision transmitter.

A still further object of my invention is to `provide an improvedtelevision receiver.

A still further object of` my invention is to provide an improved meansfor and a method of producing recurring impulses each having a frontside of different slope than its back side.

In a preferred embodiment of my invention l produce horizontalsynchronizing impulses andl `vertical synchronizing impulses whichdiffer from each other in that in one group of impulses each impulse hasa steep front side and a gradually sloping back side while in the othergroup of impulses each impulse has a steep back side and a. graduallysloping front side. For example, each horizontal synchronizing impulsemay have a steep front `while each vertical synchronizing impulse has 'asteep back side. Preferably, the two groups of impulses have the sameamplitude whereby transmitter power is conserved just asin the case offrequency separation of synchronizing impulses. i

At the receiver, after removal oi picture'signals in a conventionalmanner, the horizontal and vertical synchronizing impulses are passedthrough a differentiating circuit whereby the steep front edge of ahorizontal synchronizing impulse produces a sharp positive impulsewhilethe steep back edge of .a vertical synchronizing chronizing impulses maythen be separated from each other and impressed upon their respectivedeecting circuits for maintaining synchronism between transmitter andreceiver. Or, as will be more fully explained hereinafter, afterdifferentiation the two groups of synchronizing signals may be impressedupon the deilecting circuit oscillators without separation provided thepolarity of the synchronizing signals is reversed for one deectingcircuit. In other Words, certain oscillators, such as blockingoscillators, will synchronize only on positive impulses, the presence ofnegative impulses not being detrimental.

The invention will be better understood from the following descriptiontaken in connection with the accompanying drawings, in which Figure 1 isa block diagram of a television transmitter embodying my invention,

Figure 2 is a view of the` synchronizing and blanking impulse generatingdisc shown in Fig. 1,

Figure 3 is an enlarged view of the upper and lower sections of the discshown in Fig. 2,

Figures 4 to l1 are curves showing the shapes of the synchronizingimpulses and also showing how they are combined with the picture andblanking signals,

Figure 12 is a circuit diagram of a portion of the transmitter shown inFig. 1,

Figures 13 and 14 are circuit diagrams oiy two television receiversconstructed in accordance with different embodiments of my invention,

'Figure 15 is a circuit diagram showing one way of synchronizing adefiecting circuit in a system embodying my invention,

Figure 16 is a circuit diagram of an electronic generator for producingsynchronizing signals in accordance with my invention,

Figures 17 and 18 are curves which are referred to in explaining amodification of the invention shown in Fig. 16, f

Figure 19 is a circuit diagram showing a modiiication of a portion ofthe circuit shown in Fig. 16, and

Figure 20 is a circuit diagram of another electronic generator forproducing synchronizing signals in accordance with my invention.

Before describing the transmitter shown in Fig..l, it will be helpful torefer to Fig. 7 which shows the shape of the horizontal synchronizingimpulses indicated at H and of the vertical synchronizing impulsesindicated at V. It will be seen that each of theimpulses H has a steepfront and a, gradually sloping back While the impulse V has a graduallysloping front and a steep back. It will be noted that the H and Vimpulses are of the same amplitude and that an H impulse hassubstantially the same duration and energy content as a V impulse. Yet,at the receiver the horizontal synchronizing impulses and the verticalsynchronizing impulses may be separated if the signals shown in Fig. 7are passed through a differentiating circuit. This produces signals ofthe character shown in Fig. 10. It is apparent that here the horizontalsynchronizing impulses and the vertical synchronizing impulses are ofopposite polarity and may be separated from each other by amplitudeselection.

Referring now to Fig. 1, I have shown merely by way of example, atransmitter in which a disc I rotated by a motorv I I is employed forproducing synchronizing impulses of the desired shape. Near theperiphery of the disc I0 there are openings for producing thesynchronizing impulses. The other row of openings is for producingblanking impulses. 'I'he shapes and spacings of the openings will bedescribed later in connection with Figs. 2 and 3.

y'I'o produce the horizontal synchronizing impulses, a source of lightI2, a mask I3 having a slit therein, and a suitable optical system forma vertical slit of light on the row of synchronizing openings. As anopening rotates past this slit of light an amount of light dependentupon the shape of the opening passes therethrough and impinges upon aphotoelectric cell I4 to produce voltage impulses of the desired shapeat the input of an amplifier I6. i

Blanking impulses are produced in a similar manner by means of a lightsource I1, a slotted mask I8 and a suitable optical system which form avertical slit of light on the row of blanking openings. Thus an amountof light dependent upon the shape of a blanking opening reaches aphotoelectric cell I9 whereby voltage impulses of the desired shape andduration are supplied to the input of an amplifier 2|.

The device for generating the picture signal is illustrated as a cathoderay transmitter tube of well-known design. It comprises a highlyevacuated envelope 22 having therein a cathode 23, a grid 24, a firstanode 26 and a second anode 21 for producing an electron beam. Envelope22 also contains a mosaic 28 of lphotoelectric elements which areinsulated from each` other and from a metallic back plate 29.

An optical image of the subject to be transmitted is projected upon themosaic 28 whereby an electrostatic image is formed thereon. Thus, whenthe mosaic is scanned by the cathode' ray, picture signals are producedwhich appear across an output resistor 3| connected between the backplate 29 and the second anode 21. These pictures are then amplied by apicture amplifier 30 and supplied to a radio transmitter 31.

'I'he cathode ray may be caused to scan the mosaic 28 by means ofhorizontal and vertical deiiecting coils 32 and 33, respectively, whichare supplied with saw-tooth current from their respective deiiectingcircuits 34 and 36.

The horizontal deflection and the vertical defiection of the cathode rayof the transmitter tube are made to occur in the proper time relation bymeans of the synchronizing impulses supplied from amplifier I6 over aconductor 38 to a differentiating circuit 39 in the output of whichappear signals of the character shown in Fig. 10.

The deflecting circuits 34 and 36 may be of any suitable design, eachdeiiecting circuit including an oscillator such as a blockingoscillator. A de-v iiecting circuit of this character is described inTolson Reissue Patent 20,338.

Since a blocking oscillator is pulled into synchronism by impulses ofpositive polarity, the output of the differentiating circuit may beirnpressed directly upon the horizontal defiecting circuit whereby thepositive horizontal synchronizing impulses control the horizontaldeflection.

By passing the output of circuit 39 through a polarity inverter 4I, suchas a single amplifier tube, the vertical synchronizing impulses aregiven a positive polarity whereby they, when impressed directly upon thevertical deecting circuit, control the vertical deflection.

Suitable differentiating circuits will be described in detailhereinafter.

Blanking signals from the output of amplifier 2I are supplied over aconductor 42 to the grid 24 of the cathode ray transmitter tube forblocking the cathode ray during .the lhorizontal return period andduring the vertical return period.

Since the synchronizing and blankingfsignals are to be transmittedtogether with the picture signals to a receiverfthe blanking signals4are `supplied through an amplifier 43 to a suitable ampliiier stage inthe picture amplifier 30 for reture signals and for producing pedestalsupon which the synchronizing impulses are'` tov be located.

Thev synchronizing impulses through an amplifier 44` toa later stage inthe picture amplifier` 30 where they are added to the picture'andblanking signals.

vThe way in which picture, blanking and synchronizing signals `are addedis illustrated in Figs.,4'to 8. 'I'he picture signal appearing acrossresistor 3| is shownin Fig. 4 at 46. It is mixed with the blankingsignals shown` in Fig. 5 whereby undesired transients lli'l are removedfrom the region of the picture signals. These transients are nextclipped off to produce the signal shown are supplied moving undesiredtransient signals from the pica horizontal synchronizlngfrequency of13,230 impulses per second. On the drawings,` no .attempt has been madeto draw the disc openings in the proper scale to include all 441openings H1 v on a disc of the size shown.

Referring ,now to the openings for producing the blanking impulses ofthe character shown in Fig. 5, openings 52 and 63 are opposite openingsl openings are located slightly ahead of thesynin Fig. 6 consisting ofpicture signals 46 and pedi estals 48 and 49 which, at the receiver,provide horizontal return lineand vertical return line blanking,respectively.

The synchronizing signals shown in Fig. 7 are then added tothe signalshown in Fig. 6 to produce `the complete signal shown in Fig. 8thiscomplete signal being used to modulate the radio transmitter carrier.

The above-described method of combiningl picture signals, pedestals andsynchronizing impulses is described and claimed in my abovementionedUnited States Patent 2,192,121.

Now referring more in detail tc` the impulse generating disc I0, theshapes of the synchronizing and blanking openings and their relativelocations are shown in Fig. 2 and in theiragmen- `tary views in Fig. 3.Referring to Figs. 2 and 3,

the two diametrically opposedopeningsVi and Vn for producing thevertical synchronizing irnpulses V shownin Figs. 8 and 9 have asteepfront side and a gradually sloping back side, it being noted that thedisc lll is rotated in the direction of the arrow. "I'he slit of lightwhich is projected'upon the row of synchronizing openings is indicatedat 5i.

'I'he openings H1 which produce the horizontal synchronizing impulses Hshown in Figs. 8 and 9 chronizing openings to produce a shelf at thefront of the synchronizing impulses, as indicated at 51 and 58 in Fig.8. 1

While my invention is being described in connection with an interlacedscanning system and is especially `usei'ul in such a system forobtaining exact interlacing, it should be understood that the inventionis not limited t0 scanning of this type.

In Fig. 9 there is shown the complete signal as it occurs 1/60 secondafter the occurrence of the signal shown in Fig. 8, the steep fronts ofthe imhave steep front sides and gradually sloping back sides.

The relation between thecpenings V1, Vz-and H1 is such as to produceinterlaced scanning in the manner described and claimed in UnitedStatesPatent No. 2,152,234, issued March 28, 1939, in the name of R. C.Ballard, entitled "Television systems and assigned to the RadioCorporation of America. In my disc shown in Fig. 2, the steep edge of anopening corresponds to the center of an opening in the disc shown inthis Ballard patent.

As taught by Ballard, the interlacing is obtained by having thehorizontal synchronizing* "second are transmitted. To produce a 44llinepicture, 441 openings H1 are required, this giving pulses H in Fig. 9for the odd lines falling half way between the steep fronts of theimpulses H in Fig. 8 for the even lines.

shall now describe how the synchronizing impulses supplied iromamplifier i6 inFig, l are utilized for synchronizing purposes.

character shown in Fig. 7 and of positive polarity are supplied 'to thedifferentiating circuit 39. In the specific embodiment shown in Fig. l2,circuit 39 comprises a vacuum tube 6i which/is biased during operationto function as an amplifier. This bias may be grid leak bias maintainedby the periodically recurring horizontal synchronizing impulses whichare of positive polarity at the input of tube 6i. Or the control grid oftube iii may be properly biased by means of a biasing battery or othersuitable biasing source.

Plate voltage may b e supplied to the plate of tube 6I through a plateresistor 62.

The synchronizing impulses appearing across resistor 62, which are asshown in Fig. 7 but ampliiled vand inverted in polarity, are impressedacross a diierentiating circuit consisting of a `1 condenser 63 of smallcapacity and a resistor 64 of comparatively low resistance. Theresistance of plate` resistor 62 should be low compared with theresistance of resistor` 64 whereby the current flow through resistor 64is determined mainly by the impedance of condenser 63. The resistance of62 may be Vgn of the resistance of 64, for example. The time constant ofthe circuit 62- 63--64 should be short compared with the duration ofeach horizontal synchronizing impulse supplied `over conductor 38. Forexample, this time constant may be 1/5 of the duration of such animpulse. It will beunderstood that the values are not critical and thatexact or complete diierentiating is not required. Substantialdifferentiation is all that is required.

The resulting signal appearing across the resistor 64 is asV shown inFig. `10. The action of the differentiating circuit will `be seen bycomparing Figs. 7 and l0 which are in the same time Referring to Figs. land l2, synchronizing impulses of the relation where Fig. 7 shows thesignal which is being impressed upon the input circuit of a tube havinga diierentiating circuit in its output circuit. It will be seen that thesteep front of impulse H causes a negative impulse 66 of substantialamplitude while the gradually sloping back of impulse H causes apositive impulse 61 of small amplitude.

On the other hand, the steep back of impulse V causes a positive impulse68 of substantial amplitude while the gradually sloping front of impulseV causes a negative impulse 69 of small amplitude.

Where interlacing is employed, the impulses occurring lfm second laterare in the time relation shown in Fig. 9 and these impulses produce lacross resistor 64 the impulses shown in Fig. 11. Since, as indicated bythe legend in Fig. 12, the vertical impulses 68 have been made to appearwith a positive polarity at the high potential end of resistor 64, theymay be applied directly to the blocking oscillator (not shown) in thedeilecting circuit 36, the negative horizontal impulses having no effecton the oscillator.

As indicated at 4| an amplifier tube 69 reverses the polarity of thediiierentiated'signals whereby the horizontal impulses 66 are appliedwith a positive polarity to the blocking oscillator (not shown) in thedeiiecting circuit 34, the negative vertical impulses having no effectupon the oscillator.

In Fig. 13 there is shownv a receiver designed for the reception of thesignals radiated by the transmitter shown in Fig. l. It comprises asuitable radio receiver 1| which demodulates the receved carrier waveand supplies the composite signal shown in Figs. 8 and 9 to a cathoderayA receiver tube 12 through ampliiier tubes 13 and 14.

From the output circuit of tube 13, where the synchronizing impulses areof positive polarity, the composite signal is fed over a conductor 16 toa separating tube 11 which clips the signal at the level indicated bythe dotted line 18 inFigs.

8 and 9 to pass only the synchronizing impulses.

Separating tube 11 preferably is grid leak ,biased as described andclaimed in United States Patent No. 2,207,839, issued July 16, 1940, inthe name of W. A. Tolson, and assigned to the Radio Corporation ofAmerica.

In this embodiment of the invention the plate circuit, of tube 'l1includes an inductance coil 19 which, at the highest frequencycomponents of the synchronizing impulses, has an impedance which is lowcompared with the plate impedance of tube 11. With this impedancerelation of coil 19 and tubes 11, the current ow through coil 19 issubstantially independent of the coil impedance whereby the currentimpulses through coil 19 have the same wave shape as the voltagesynchronizing impulses on the control grid of tube 11. y f

Since the voltage appearing across coil 19 is equal to di Ld: f

where L is the coil inductance and where i is the current flowingthrough the coil, it is apparent that the voltage across coil 19 is thederivativeA of the voltage impressed upon the control grid of tube 11. Y

Thus, impulses like those shown in Figs. 10 and 11 are supplied to thedeflectlng circuits. In the vertical `deilecting circuit the positivevertical impulses 68 (Figs. l0 and 11) are impressed directly upon thegrid circuit of a blocking oscillator 8| which is part of the verticaldetlecting circuit.

The oscillator 8|, which is adjusted to oscillate uncontrolled at aslightly lower frequency than the rate at which impulses 68 occur, ispulled in step by impulses 68 whereby sharp negative impulses of largeamplitude are supplied to a suitable saw-tooth current generator 82which makes up the remainder of the vertical deiiecting circuit forcausing a flow of saw-tooth curernt through vertical deecting coils 83.

I'he negative synchronizing impulses 66 (Figs. 10 and 11) are reversedin polarity by ampliiier tube 61 whereby positive horizontalsynchronizing impulses are supplied to the blocking oscillator 84 of thehorizontal deflecting circuit. Oscillator 84 impresses sharp negativeimpulses upon the saw-tooth current generator 86 to cause a ilow ofsaw-tooth current through the horizontal deflecting coils 81.

The above-described vertical and horizontal deflecting circuits may bedesigned in accordance with theteachings of Tolson Reissue Patent 20,-338 and/or United States Patent No. 2,101,520, issued December 7, 1937,as a joint invention of W. A. Tolson and J. R. Duncan, and assigned tothe Radio Corporation of America.

It Willbe understood that my invention is not limited to any particulartype of deflecting circuit. For example, a multivibrator or a gas tubeoscillator may replace the blocking oscillator, or the synchronizingimpulses may be amplified sufciently to be impressed directly upon thesawtooth generator without the use of an oscillator. Obviouslyelectrostatic deflection may be utilized inr place of electromagneticdeflection.

In some cases it may be desirable to isolate the horizontalsynchronizing impulses 66 and the vertical synchronizing impulses 68from each other before they are supplied to their respective deflectingcircuits. This may be done as shown in Fig. 14, Where partscorresponding to those in Fig, 13 are indicated by the same referencenumerals;

In this circuit the separating tube operates the same as the separatingtubes shown in Figs. 12

and 13 while the differentiating circuit is the 'Y same as shown in Fig.12, comprising the small capacity condenser 88 and the resistor 89.

The differentiated signals, having the polarity indicated by the legend,are suppliedy to a negatively biased clipping tube 9i in the verticaldeflecting channel whereby only the positive vertical synchronizingimpulses appear in the clipping tube output circuit. These impulses arethen reversed in polarity or inverted by a tube 92 whereby they aresupplied with a positive polarity -to the deecting circuit 8 l-82.

In the horizontal deflecting circuit, the differentiated signals areinverted by a tube 93 whereby positive horizontal synchronizatingimpulses are supplied to a clipping tube 94. Only horizontalsynchronizing impulses appear in the output of tube 94. As they are ofnegative polarity, they are inverted by a tube 96 before being impressedupon the deflecting circuit 84-86.

It will be understood that the diiferentiated synchronizing impulses maybe applied in various ways to an oscillator for pullingit intolsynchronism. For example, as shown in Fig. 15, a blocking oscillator ofthe type described in the above-mentioned Tolson patent may comprise ascreen grid tube 91 having its cathode control1 grid' and p1ate``connected "in 'the `usual way to `form av blocking oscillator.' Thescreen 'n'grid 98 is utilized forsynchronizing purposes, the

diierentiated'synchronizing impulses being imsigned to the RadioCorporation of America. As"

described in the said patent, a main oscillator |00, operating at doublethe horizontal scanning frequency, supplies voltage impulses to a chainof frequency dividers |02 and |03 to produce.`

rectangular impulses occurring at the picture frame frequency of 60 persecond as indicated at |04.

The main oscillator |00 also supplies voltage impulses to an oscillator|06 which supplies rectangular voltage impulses, indicated at |01,occurring at one half the main oscillator `frequency. In the generatorillustrated, the horizontal and vertical synchronizing impulses occur atthe proper frequencies to produce a 44l-line interlaced picture.

The several oscillators may beof any suitable type such asmultivibrators or blocking oscillators. y

The desired horizontal synchronizing impulses having steep front and`gradually sloping backs are produced as follows:

The impulses |01 are passed through a differentiating circuit comprisinga small capacity condenser |08 and a resistor `|09 in series," theresistor |09 being connectedas the grid leakhof a vacuum tube Theresulting voltage wave appearing on the grid of tube is shown at ||2. li

The tube is so biased and the amplitude of the input signal ||2 is suchthat only the positive polarity impulses occurring at the rate of 13,230per second appear in the plate circuit of tube their polarity beinginverted as indicated at I I3. l

Impulses` I3 are impressed upon the grid of Va vacuum tube |I4 toproduce a saw-tooth voltage comparatively large capacity connectedacross the plate circuit. The saw-tooth voltage appearing acrosscondenser H6 is indicated at ||1. It will be seen that the voltage wave||1 is produced by the` condenser H6 being charged through the plateresistor H8` while vthe tube ||4 is driven to cut-olf `byimpulses H3,and

being discharged through tube m `during the remaining period. It will beunderstood that the.v 60 saw-tooth voltage I|1 may be produced inotherwell-known ways.

In accordance with my invention the `voltage wave ||1 is supplied to theinput of a vacuum tube ||9 which is properly biased to pass only thevpeaks ofthe wave ||1. That is, the wave |11 is cupped at the levelindicated by the dotted line |2I, to produce in the plate circuit oftube 9 the voltage impulses indicated at |22.` It will be seen that theyare recurring impulses ing steep fronts and sloping backs.

The tube H9, like the other clipping tubes in the circuit, may besuitably biased by a battery the' ieaerfmeesef 'Basins 'Being employedin, the

electronic generator4 illustrated. k

Preferably. the peaks ofithe impulses |22 Vare clppedoff bygmeansY ofanother clipping `tube |23 to produce the final horiz` ntal synchronizving,` impulses indicated at H.

TheGO-cycle rectangularA impulses |04 supplied from oscillator |03 'arepassed through a diiferentiating circuit vcomprising a small capacitycondenser |26 and ajresistorf |21 whereby the voltage impulses indicatedat |28 are impressed upon the grid of a clipping tube |29.' Only thepositive impulses appear in the plate circuit of tube |29 and these areinverted by the vacuum tube |3|` to produce the 'impulses indicated at|32 occurring at the rateof 60 per second.

The positive impulses |32 are impressed upon the grid of a vacuum tube|33 `having a condenser |34 of comparatively large capacity connectedacross its plate circuit. As a result, the saw-tooth voltage indicatedat appears across the condenser I34. It will be noted that this sawtoothvoltage has a sloping front and a steep back," just the opposite of thesaw-,tooth ||1 produced in the other branch of the circuit.

The saw-tooth voltage |36 is produced by the condenser |34 being chargedthrough the plate 30 resistor |31 between the positive impulses |32,

during which timetube |33 is biased substantially to cut-ofi' by gridleak biasing, and then being discharged through tube |33 for the dura`tion of a positive `impulse |32. Of course,` a

suitable saw-tooth voltage may be produced in any well-known manner.

The saw-tooth wave |36 is impressed upon a tube |30which is biased toclip the wave at the level indicated by the dottedline |39, thus 40passing only the positive peak of each sawtooth. `The resulting-Iimpulses which appear in the plate circuit of tube |38 are indicated at|4I. It will be noted that their steep side is on the back of the waverather than on the front of the Wave, as in the case of the horizontalimpulses |22.

As in the case of the horizontal impulses, the peaks of the verticalsynchronizing impulses |4| are preferably clipped off by a clipping tube|42 to produce the ilnal vertical synchronizing ine-its plate circuitdue to a condenser ||6 of impulses indicated at V.

It maybe notedthat the synchronizing impulses H and V are not drawn tothe same scale and that, because of this, the impulses V appear muchcloser together as compared with the spacing of the impulses H than theyactually are.

The impulses H and'V are added or combined by providing the tubes |23and |42 with a common plate resistor |43. The resulting signal isindicated at |44 where the combined impulses H and V are shown insubstantially their correct time relation. It will be understood that anamount, if necessary, as taught by United States Patent No. 2,132,655,issued `October 11, 1938, in the naine of J. P. Smith, and assigned tothe Radio Corporation of America. having a time interval between themand hav- Since in the above-described method of producing synchronizingimpulses only a very small (not shown) or byemeans of grid leak biasing,through the clipping tube. This. is especially The verticalsynchronizing impulses are 'produced as follows:

true in the case of the vertical impulses where only about lzoo of asaw-tooth is utilized.

Less accuracy in clipping the horizontal sawtooth voltage is required ifthe coupling oondenser |46 between tubes ||4 and ||9 is made of suchsmall capacity that the low frequency components of the saw-tooth aregreatly attenuated. 'This causes a voltage of the wave A shape shown inFig. 18 to be supplied to clipping tube ||9 which clips the wave at thelevel indicated by the dotted line |41.

In producing the vertical synchronizing impulses, the saw-tooth may b edistorted as shown in Fig. 17 by accentuating the low frequencycomponents of the saw-tooth.` This may be done by inserting betweentubes |33 and |38 at the point indicated by the dotted line |30 thecircuit shown in Fig. 19. Referring to Fig. 19, the sawtooth waveappearing across condenser 134 (Fig. 16) is impressed upon an ampliilertube |40 which has in its plate circuit a illter comprising a resistor|48 and a condenser |49. The amplifier tube |50 inverts the saw-toothwave whereby the tube |30 (Fig. 16) clips'it at the level indicated bythe dotted line in Fig. 17. The low fre-` quency components may beaccentuated the desired amount by making condenser |49 of the propercapacity.

'I'he desired blanking signals for the television system may be providedby suitable delay and shaping circuits represented by the blocks |52 and|53. Such circuits are described in the above-identied Smith patent. Y

Another suitable electronic generator for producing synchronizingsignals in accordance with my invention is shown in Fig. 20. Thisspecific generator is described and claimed in United States Patent No.2,166,688, issued July 18, 1939, in the name of R. D. Kell, entitledTelevision apparatus, and assigned to the Radio Corporation of America.

Referring to Fig. 20, there is shown another generator for a televisionsystem which produces a 441line interlaced picture. It will beunderstood that the apparatus shown in Fig. 20, like the generator shownin Fig. 16, is located at the television transmitter and supplieshorizontal and vertical synchronizing impulses to the defleetingcircuits of both the cathode ray transmitter tube and the cathode raytube at the receiver.

'Ihe horizontal and vertical synchronizing impulses are derived from acommon source such as a multivibrator or other suitable oscillator 20|.A second multivibrator 202 divides the frequency of the oscillator 20|by two to produce impulses at the line frequency, 13,230 cycles persecond in the specific system'being described.

A chain of multivibrators 203,204 and 206 divide the' frequency ofoscillator 20| to produce impulses occurringat the picture framefrequency of 60 per second. Itwill be understood that the 60-cycle and13,230-cycle impulses having the desired odd-line relationship forinterlacing may be obtained in any other suitable way as by means of arotating disc having suitable openings therein, as dscribed and claimedin Patent 2,152,234, issued March 28, 1939, inthe name oi' R. C.Ballard, entitled Television systems, and assigned to the RadioCorporation of America, or by means of blocking oscillators as describedand claimed in Patent 2,178,218, issued October 31 1939, in the name ofA. V. Bedford, entitled Television systems, and assigned tothe RadioCorporation of America.

The portion of the circuit which produces the horizontal synchronizingimpulses will first be described. The saw-tooth voltage wave, which isto be combined with a rectangular impulse, is produced in this speciilcexample by supplying a rectangular voltage impulse indicated at 205 fromthe oscillator 202 over a conductor 201 and through a resistor 203 to acondenser 209. 'I'he condenser 209 is connected across the inputelectrodes of a screen grid tube e and is shunted by a resistor 2| theresistor 2|| having a high resistance, such as one megohm, as comparedwith the resistance of resistor 208. In practice, the resistor 208 maybe the plate impedance of one of the multivibrator tubes.

Condenser 209 is charged suddenly through resistor 200 and dischargedgradually through resistor 2| whereby a saw-tooth voltage wave indicatedat 2|2 is impressed upon the control grid of tube e. Stated diierently,the circuit 209, 2|| integrates the impulses 205 to produce the sawtoothwave 2|2.

The rectangular voltage which is to be combined with the saw-tooth waveis produced, in this specic example, by supplying rectangular voltageimpulses indicated at 2|0 from oscillator 202 to a vacuum tube c througha diierentiating circuit comprising a condenser 2| 3 of small capacityand a resistor 2|4 of comparatively low resistance, the resistor 2 4being connected across the input electrodes of the tube c. It will beunderstood that impulses 2 |0 occur at the rate of 13,230 per second. Adifferentiating circuit of this character and its action is describedand claimed in the above-mentioned Smith Patent No. 2,132,655.

'I'he derivative of impulse 2| 0 appears on the grid of tube c asindicated at 2|6.

Since tube c is adjusted to act as a clipping tube, only the upperimpulse portion of wave 2||` passes through the tube. This portionappears inverted in the plate circuit of tube c, as indicated at 2|1,and is impressed upon a clipping tube d whereby a square-topped voltagewave appears in the output of tube d as indicated at 2|0. It will benoted that, by differentiating and clipping, the impulse 205 has beennarrowed but not delayed.

Impulses 2|8 are combined with the saw-tooth waves 2|2 after impulses2|8 have been suitably delayed in a delay network 2|9 to make the twogroups of signals combine in the correct time relation. The desiredaddition of signals is obtained by impressing the delayed impulses 2|8upon the screen grid of tube e. 'I'he combined signal, after beinginverted by a tube f, is of the character indicated at 22|.

Signal 22| is supplied to a clipping tube g, which clips signal 22| atthe level indicated by the dotted line to produce the desired impulse,indicated at 222, having a steep front side and a gradually sloping backside.

'The delay network 2|9 may be of any suitable design such as aconventional capacity-inductance network of recurrent sections, or itmay be a network comprising vacuum tubes and wave shaping networks oi'the character described in the above-mentioned Smith United StatesPatent No. 2,132,655. In certain cases it will be preferable to insertthe delay network 2|9 ahead of tube c in order that the delay networkwill not injure the wave shape obtained by the clipping action of tubesc and d.

Referring now to that portion of the circuit which produces the verticalsynchronizing impulscs having a steep back side and a sloping frontside, rectangular voltage impulses occurring at the framing frequency of60 cyclesper` second and indicated at 225 `are supplied from oscillator206 over a conductor 226, through -a resistor 221 to a `condenser' 228which is connectedacross the input electrodes of vacuum tube e.Condenser 228 is shunted by a resistor 229having comparativelyhighresistance. Thus, asaw-tooth voltage wave is suppliedto the grid of tubee as indicated at 23l.- It will be seen that tube ef corresponds to tubee in the horizontal synchronizing circuit.

e Similarly, the vacuum tubes c. d', `f and g" now to be describedcorrespond to tubes c, d, f and g in the. horizontal synchronizingcircuit. e

The rectangular voltage impulse to be added to the saw-tooth 23| isobtained by taking from oscillator 206 a. rectangular voltage impulseindicated at 232,whichoccurs at the frame frequency i of 60 per second,`and supplying it to a differenafter further clipping by tube `d appearsin the plate circuit of tube d' as a narrowed rectangular impulseindicated at 231.

i Impulses 231 are delayed by a delay network 238 which may be of thesame type as delay net- 'work 2|9. The delayed impulses 231 are addedthat impulses 231 are delayed is such as to make them add on the frontside of the saw-tooth wave rather than on the back side as is the casein the horizontal synchronizing circuit. Thus. after being inverted bytube f', the combined signal appears in the output circuit of tube j',as indicated at 239. This signal is then clipped bythe tube g', asindicated by the dotted line, to produce the desired signal having asteep back side and a sloping front sideyas shown at 24|.

The vertical synchronizing signals 24| and the horizontal synchronizingsignals 222 are com bined` in the common plate circuit of tubes g andg', the combined signals then have `their sharp peaks/clipped oil byaclipping tube h whereby there appears in the output circuit 'of tube hthe desiredsignal of the character shown at 242.

From the foregoing description, it will be apparent that variousmodifications may be made in my invention without departing from thespirit and scope thereof and I desire, therefore, that only suchlimitations shall be placed thereon as are necessitated by the prior artand are set forth in the appended claims.

I claim as my invention:

1.` In a picture` transmitting system, the method of synchronizingscanning at the receiver with scanning at the transmitter whichcomprises transmitting one group of synchronizing impulses each having asteep front side and having a gradually sloping back side andtransmitting another group of synchronizing impulses each having a steepback side and a gradually sloping front side; intercepting said impulsesat the receiver and substantially i diierentiating them, and utilizingthe differentiated signals for synchronizing the scanning at thereceiver.

2. The method of `synchronizing the scanning of a view to be transmittedwith the scanning at a receiver for receiving said view which compulsesoccurring at a relatively high frequency and, transmitting a grcupofsynchronizing impulses occurringat a relatively low frequency, the

impulses'in `one `of said groups each having a -1 steep front side and`a Ygradually sloping back side,`the impulses in `the other of saidgroups each having a steep lback side and a gradually sloping frontside, intercepting both groups of said impulses at said receiver andsubstantially differentiating them whereby the steep sides of saidimpulses produce impulses of greater amplitude than the graduallysloping sides of said impulses and whereby the said impulses of greateramplitude produced by one of said groups of impulses are of oppositepolarity to the said impulses of greater amplitude producedby the otherof said groups of impulses, andutilizing the said impulses of greateramplitude for synchronizing the scanning atgsaid receiver.

3. In a picture transmitting system, a picture transmitter includingmeans `for transmitting picture signals, means for generating andtransmitting a group of recurring synchronizing impulses each having. avfront side which is'` steep as compared withits backside, means forsimultaneously generating and transmitting another group of recurringsynchronizing impulses each having a back side which is steep ascompared with its front side, a picture receiver for receiving saidpicture signals andsaid groups of impulses, said receiver includingmeans for differentiating said impulses whereby impulses of maximumamplitude and of a certainV polarity are derived from one of said groupsand wherebyimpulses of maximum amplitude and of the opposite polarity tosaid certain polarity are derived from the other of said groups, andmeans for utilizing said im- 4 pulses of maximum amplitude forsynchronizing the' scanning at the receiver with the scanning at thetransmitter.

4. A generator of synchronizing signals comprising means for producing agroup of saw-tooth Waves occurring at a comparatively high frequency,each of said waves having a front slope which differs irom its backslope, means for clipping said waves near their peaks and passing saidpeaks to an output circuit, means for producing another group ofsaw-tooth waves occurring at a comparatively low frequency. each of saidlastmentioned waves having a front slope which differs from its backslope, the ratio of the front slope to theback slope of the waves in oneof said groups being greater than unity and the ratio` ofthe front slopeto the back slope of the waves in the other of said groups beinglessthan unity, and means for clipping said low frequency waves near theirpeaks and passing said peaks to said output circuit. i

5. In a television transmitting system, means for developing picturesignals, means for developing line synchronizing signals having aleading edge of predetermined slope and a trailing edge of a differentslope, means for developing a. frame synchronizing signal having leadingand trailing edges'of different slopes, the slope of the leading andtrailing edges "of the frame synchronizing signal being different fromthe leading and trailing edges of the line synchronizing signal, and

prises transmitting a group of. synchronizing `iment slope, means fordeveloping a frame synchronizingsignal having leading and trailing edgesof diil'erent slopes which are diierent from the slopes of the leadingand trailing edges of the line synchronizing signal, means for combiningthe picture, line and frame synchronizing signal, means for transmittingthe combined wave, comprising means for developing and transmitting saidcarrier wave comprising means for generating a video-frequency wave ofone polarity, means for developing a composite synchronizing signal ofthe other polarity including line-synchronizing and eld-synchronizingimpulses having ieading and trailing edges of a predetermined differencein slope, the corresponding diierences in slope of theline-synchronizing and held-synchronizing impulses being opposite insense. means for modulating said carrier with said video-frequency waveand said composite synchronizing signal, means for receiving andutilizing said carrier comprising amplitudelimiting means for separatingsaid synchronizing impulses from said composite signal, and means forderiving from the leading edges of one type of synchronizing impulsescharacteristic\synchronizing pulses and for deriving from the vtrailingedges of the other type of synchronizing vimpulses differentcharacteristic synchronizing pulses.

8. A television synchronizing system, comprising means for developingsynchronizing impulses of a first predetermined frequency and havingleading edges of steeper slope than their trailing edges, means fordeveloping synchronizing impulses of a second predetermined frequencyand having trailing edges of steeper slope than their leading edges, oneof said predetermined frequencies beinga fractional multiple of theother, and means for combining said synchronizing impulses of both ofsaid frequencies to develop a composite synchronizing signal.

' ALDA V. BEDFORD.

